Earnings per Share - Net Income Attributable to the Company, Common Shares Outstanding, and Participating Securities Outstanding (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Net income	$ 70,448	$ 53,344
Less: Net income attributable to non-controlling interests	17,323	12,510
Net income attributable to Select Medical Holdings Corporation	53,125	40,834
Basic EPS
Less: net income attributable to participating securities	1,818	1,343
Net income attributable to common shares	51,307	39,491
Diluted EPS
Less: net income attributable to participating securities	1,818	1,343
Net income attributable to common shares	$ 51,307	$ 39,491